Albanian politicians will suffer 'severe, long-lasting' consequences from Washington, the U.S. ambassador said on Tuesday, if they block a reform of the justice system that would open the way to talks on joining the European Union.

Despite 18 months of talks, the main political parties have so far failed to reach a deal on overhauling the judiciary to make it independent and capable of fighting endemic corruption in the former communist Balkan country.

If they do not deliver the reform by Thursday, NATO member Albania will lose the chance to open negotiations this year on joining the EU - a transformative step for a country that was virtually cut off from the outside world during decades of Stalinist rule until 1990.

Joining protesters outside parliament demanding the passage of the reform, U.S. envoy Donald Lu blamed the delay on "powerful people afraid" of the changes, which according to an opinion poll are supported by 90 percent of Albanians.

A key sticking point is the role of EU and U.S. officials in the body that will vet judges to root out corrupt ones. Some opposition figures see this as a breach of sovereignty.

"There will be specific negative consequences from the United States for political leaders who vote against this reform," Lu told the rally, amid applause.

"I am delivering details of these consequences to political parties on specific instructions from Washington. I can assure you that these consequences will be severe and long-lasting," he added, without saying what they would be.

EU Enlargement Commissioner Johannes Hahn said: "We want to make sure that the legislation has teeth." He said the bloc wanted to see the reform being implemented before embarking on membership talks.

Ideally, the reform should pass with broad consensus to give it rock-solid backing, but Prime Minister Edi Rama said 94 votes in the 140-member chamber would do, as the constitution stipulates a two-thirds majority.

His coalition lacks that many votes, but he could get those from the opposition if the U.S. pressure bears fruit.

Opposition Democratic Party leader Lulzim Basha and Rama's coalition partner are working to achieve consensus down to the last minute, warning that failure would be destructive.